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We report the confirmation of an old, metal-poor globular cluster in thenearby dwarf irregular galaxy Sextans A, the first globular cluster known inthis galaxy. The cluster, which we designate as Sextans A-GC1, lies some 4.4arcminutes ($\sim1.8$ kpc) to the SW of the galaxy centre and clearly resolvesinto stars in sub-arcsecond seeing ground-based imaging.We measure anintegrated magnitude $V=18.04$, corresponding to an absolute magnitude,$M_{V,0} = -7.85$. This gives an inferred mass $M\sim$1.6$\times10^5~M\odot$,assuming a Kroupa IMF. An integrated spectrum of Sextans A-GC1 reveals aheliocentric radial velocity $v_{\rm helio}=305\pm15$~ km/s, consistent withthe systemic velocity of Sextans A. The location of candidate red giant branchstars in the cluster, and stellar population analyses of the cluster'sintegrated optical spectrum, suggests a metallicity [Fe/H] $\sim$--2.4, and anage $\sim9$ Gyr. We measure a half light radius, $R_h = 7.6\pm0.2$ pc.Normalising to the galaxy integrated magnitude, we obtain a $V$-band specificfrequency, $S_N=2.1$. We compile a sample of 1,928 GCs in 28 galaxies withspectroscopic metallicities and find that the low metallicity of Sextans A-GC1is close to a "metallicity floor" at [Fe/H] $\sim-2.5$ seen in these globularcluster systems which include the Milky Way, M31, M87 and the Large MagellanicCloud. This metallicity floor appears to hold across 6 dex in host galaxystellar mass and is seen in galaxies with and without accreted GCsubpopulations.

An old, metal-poor globular cluster in Sextans A and the metallicity floor of globular cluster systems